1. Field of the Invention
The present invention relates to the improved utilization of phosphorus pentasulfide in thionylations using phase transfer catalysis. More particularly, the present invention relates to the conversion of S-thiol esters of benzo[b]thiophenecarboxylic acids to the corresponding dithio esters using reduced amounts of phosphorus pentasulfide in the presence of a phase transfer catalyst and catalytic amounts of water. By using less P2S5, the environmental impact is reduced when the spent P2S5 is processed for disposal. There is also a positive financial impact when less raw material is used in a chemical process.
2. Description of Related Art
2-Benzo[b]thiophenecarbodithioic esters are important intermediates in the manufacture of material preservation chemicals. See, e.g., EP 0715625.
U.S. Pat. No. 5,965,749 discloses a process for the preparation of a substituted 2-benzo[b]thiophenecarboxylic acid compound or an alkali metal salt thereof, which comprises reacting a halobenzaldehyde derivative with mercaptoacetic acid in the presence of an alkali metal hydroxide and water.
Notwithstanding that esters are not readily converted to their corresponding dithioesters, in U.S. Pat. No. 5,965,749,2-benzo[b]thiophenecarboxylic acids were first converted to a S-thiolester and subsequently reacted with 1.5 equivalents of P2S5 in hot toluene to give the corresponding carbodithioic ester. It has been found that it was important that a highly active grade of P2S5 was necessary in order for the reaction to proceed to completion. Nonetheless, this thionylation required an excessive amount of P2S5 and consequently, a corresponding amount of spent P2S5 required particular care in its destruction and subsequent disposal.
It is known that P2S5 can be used to convert aromatic carboxylic acids in the presence of alkyl thiols to the corresponding dithioesters. Sudalai et al., Org. Lett. 2(20):3213-3216, (2000), teaches this conversion using high boiling alkyl thiols and heating the reactants for extended periods of time. This failed to work for 2-benzo[b]thiophenecarboxylic acid.
Davy et al., Chemistry and Industry, page 824, December, 1985 teaches a similar conversion to dithiocarboxylic esters, whereby carboxylic acids and P2S5 in a suitable alcohol in 1,2 dichlorobenzene as solvent were mixed and subsequently heated to 178° C. This method also did not work with 2-benzo[b]thiophenecarboxylic acid.
Rao et al., Synthetic Communications, 31(22):3469-3472 (2001) used excess P2S5, potassium carbonate, and benzyltriethylammonium chloride (TEBA) in dichloroethane to convert a lactam to the corresponding thiolactam. Despite the use of phase transfer catalysts in Rao et al's work, no advantage was expected over that described in thionylations outlined in U.S. Pat. No. 5,965,749.